In ITS (Intelligent Transport Systems), systems for providing information which is useful for avoidance of an accident such as early detection of an oncoming car on a road of poor visibility and advance notification of an incoming car to an impenetrable intersection are being investigated. Also in ITS, service for providing various kinds of information useful at the time of transfer such as traffic guidance and congestion information is also being investigated. As a means for realizing such system, there has been a proposal of structure in which a terminal apparatus capable of wireless communication into each car is installed, for sending and receiving various kinds of information by the terminal apparatus. A terminal apparatus for short-range communication (DSRC: Dedicated Short-Range Communication) used in ITS is disclosed in patent document 1, for example.
In ITS, when various kinds of information are sent and received (usually, by packet communication) between the terminal apparatuses installed into respective cars, information is transmitted by relatively large constant electric power so that communication may be possible even for a car located at an edge of a target communication area. Here, a target communication area, which is specified in ITS, indicates an area (a distance) in which wireless communication under a within-the-horizon condition (within-the-horizon communication) is possible and an area (a distance) in which wireless communication under a beyond-the-horizon condition (beyond-the-horizon communication) is possible.
Meanwhile, an electric power control method for controlling transmission power of a mobile station in a wireless LAN system is described, for example in patent document 2 or patent document 3, although that has not been applied to ITS.